Method for attaching an integrated circuit package to a circuit board

ABSTRACT

A method for attaching an IC package to a circuit board, the IC package having a plurality of electrical contacts in an arrangement having a perimeter, first positions the IC package adjacent to the circuit board. Then, electrically connects the IC package to the circuit board through the plurality of electrical contacts. The method finally, disposes at least one anchor mechanically attaching the IC package to the circuit board, the anchor disposed at a location outside of the perimeter of the plurality of electrical contacts. The type, quantity, and exact geometry of the anchors depend on the specific design parameters of the IC package and circuit board.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/064,462,filed Jul. 17, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an integrated circuit (IC) package, andmore specifically to a method for attaching an IC package to a circuitboard.

2. Description of the Prior Art

An integrated circuit is typically housed inside of a package made ofceramic or composite material. The package provides mechanical,electrical, and thermal protection to the IC chip. The package hascontacts to which solder balls or pins are attached to allow the IC chipto be electrically connected to a circuit board, and at the same time toprovide mechanical attachment of the package to the circuit board. Thesolder balls or pins serve the purpose of both electrical and mechanicalconnection.

FIG. 1 shows a bottom view of a prior art IC package 10 having an ICchip (item 16 of FIG. 2). A plurality of solder balls 12 is disposed ona bottom surface of the package 10 in a grid-like array for attachingthe package to a circuit board.

A prior art method for attaching the IC package 10 to a circuit board 14to manufacture an assembly 18 is illustrated in a side view shown inFIG. 2. The IC package 10 holding the IC chip 16 is attached to thecircuit board 14 by the plurality of solder balls 12. Duringmanufacture, the solder balls 12 are fused to both the package 10 andthe circuit board 14 to form an electrical and mechanical connection.

The solder balls 12 can fail under stress from thermally inducedmechanical loading or direct mechanical loading of the assembly 18. Forinstance, the package 10 and circuit board 14 typically have differentcoefficients of thermal expansion. As the assembly 18 is heated orcooled the circuit board 14 and package 10 expand or contract atdifferent rates, and consequently the solder balls 12 can be subject tohigh stresses. Additionally, the circuit board 14 can be flexed byexternally applied forces, such as those forces experienced during themanufacturing, assembly, and testing processes. However, because themechanical stiffness of the IC package 10 is appreciably greater thanthat of the circuit board 14, the solder balls 12 can be subject to highstresses that can also cause mechanical failure. While the principalstresses in the solder balls 12 under these loading conditions may notbe enough to cause immediate failure, a cyclic mechanical or thermalload can cause a fatigue failure.

FIG. 3 shows the assembly 18 bending about a single axis due to adifferential thermal contraction of the circuit board 14 and the package10. This condition can be caused, for example, by the assembly 18 beingcooled from a higher than ambient assembly temperature resulting in thecircuit board 14 contracting more than the package 10. As the assembly18 cooled, solder balls in two rows of solder balls 12 a and 12 b weresubject to excessive tensile stress, and consequently cracked as shown.After a solder ball has failed mechanically, the IC package 10 is nolonger effectively electrically connected to the circuit board 14.Furthermore, if the loading on the circuit board 14 is not somehowreleased, the package 10 may become completely mechanically separatedfrom the circuit board 14.

The prior art method of using solder balls to provide a mechanicalconnection between an IC package and a circuit board results in a weakconnection that is prone to mechanical failure. When the circuit boardand package undergo a mechanical loading or thermally induced mechanicalloading, solder balls can fail mechanically.

SUMMARY OF THE INVENTION

It is therefore a primary objective of the claimed invention to providea method for attaching an IC package to a circuit board, so that themechanical attachment strength is increased and the problems of theprior art are solved.

According to the claimed invention, a method for attaching a IC packageto a circuit board, the IC package having a plurality of electricalcontacts in an arrangement having a perimeter, first positions the ICpackage adjacent to the circuit board. Then, electrically connects theIC package to the circuit board through the plurality of electricalcontacts. The method finally, disposes at least one anchor mechanicallyattaching the IC package to the circuit board, the anchor disposed at alocation outside of the perimeter of the plurality of electricalcontacts.

The type, quantity, and exact geometry of the anchors depend on thespecific design parameters of the IC package and circuit board.

It is an advantage of the claimed invention that the anchors can preventmechanical failure of the electrical connection of the IC package to thecircuit board, caused by mechanical and thermally induced mechanicalloading.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom view of a prior art IC package.

FIG. 2 is a side view showing a prior art method for attaching the ICpackage of FIG. 1 to a circuit board.

FIG. 3 is a side view of the assembly of FIG. 2 bending about a singleaxis.

FIG. 4 is a perspective view of attaching an IC package to a circuitboard according to the present invention.

FIG. 5 is a bottom view of the IC package of FIG. 4.

FIG. 6 is a side view of the assembly shown in FIG. 4.

FIG. 7 is a side view of the assembly of FIG. 4 bending about a singleaxis.

FIG. 8 is a side view of a second embodiment of the present invention.

FIG. 9 is a side view of a third embodiment of the present invention.

FIG. 10 is a bottom view of a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention method will be described in a preferred embodimentwhere four pins are used to strengthen the mechanical attachment of anIC package to a circuit board. Three alternative embodiments will alsobe described, wherein leads and solder are used as means forstrengthening the attachment. The teachings of the present invention canapply to many modern IC package attachment systems.

A perspective view of the preferred embodiment method of attaching an ICpackage 20, accommodating an IC chip 26, to a circuit board 22 tomanufacture an assembly 24 is shown in FIG. 4. The package 20 is firstaligned with the circuit board 22. The package 20 is then electricallyconnected to the circuit board 22 by solder balls (item 30 of FIG. 5).The package 20 is further mechanically attached to the circuit board 22by a group of four anchor pins 28. As the assembly 24 is manufactured,the anchor pins 28 can be produced at the same time and by the samedevice, as the electrically connecting solder balls. The pins 28 providefor a strengthened mechanical attachment of the package 20 to thecircuit board 22.

FIG. 5 is a bottom view of the IC package 20 showing a plurality ofsolder balls 30 disposed on a bottom surface of the package 20 in agrid-like array for electrically connecting the package to a circuitboard 22. Each anchor pin 28 is located at a corner of the package 20.In practical application, the anchor pins 28 are fit into holes formedin the package 20, but the anchor pins 28 could also easily be fuseddirectly to the bottom of the package 20 to have the same anchoringeffect. The anchor pins 28 are arranged at the four corners of thepackage 20 outside of a perimeter formed by the outmost solder balls 30,as this placement ensures that the anchor pins 28 will support asignificant portion of any loading applied to the package 20 or thecircuit board 22. Alternatively, the anchor pins 28 could beintermingled with the solder balls 30, but this arrangement is lessdesirable since the solder balls 30 are typically clustered togetherquite closely. The present invention is not limited by the arrangementof the solder balls 30 shown in FIG. 5. Moreover, the solder balls 30could be pins or other means for providing electrical connectionsbetween the package 20 and the circuit board 22.

The exact diameter, length, and material of the anchor pins 28 aredetermined based on the precise arrangement of and material used for thesolder balls 30, as well as on other design parameters. The anchor pins28, however, do not provide an electrical connection between the ICpackage 20 and the circuit board 22. For the preferred embodimentdescribed herein, the anchor pins 28 have a diameter about equal to anaverage diameter of the plurality of solder balls 30, and are of amaterial having a considerably higher allowable stress and stiffnessthan the material of the solder balls 30.

A side view of the assembly 24 is shown in FIG. 6. The anchor pins 28are illustrated penetrating both the IC package 20 and the circuit board22. In the preferred embodiment, each anchor pin 28 has a beveled headat the package end and a non-removable flattened fastener at the circuitboard end. As mentioned previously, the exact shape, size, and endconnections of the pins 28 are determined at design time considering allof the necessary factors. Essentially, the pins 28 are designed toanchor the package 20 to the circuit board 22.

Also shown in FIG. 6 is the plurality of solder balls 30 providing theelectrical connection of the IC package 20 to the circuit board 22, thedesign of such connection being well known in the art.

If the assembly 24 is subject to thermally induced mechanical loading ordirect mechanical loading, both of which may be simultaneous and cyclicas previously described in detail for the prior art, the assembly 24 maybend about a single axis as shown in FIG. 7. Certainly, the assembly 24may bend about multiple axes but for the sake of clarity, only singleaxis bending will be addressed in the description of the preferredembodiment herein. Consideration of multiple axes of bending can beaccomplished by the well-known principle of superposition. Due to adifference in coefficient of expansion, if the assembly 24 undergoesthermally induced mechanical loading, or due to a difference instiffness, if the assembly 24 undergoes direct mechanical loading, thecircuit board 22 bends significantly more than the IC package 20.Regardless of the cause of the bending, the solder balls 30 and anchorpins 28 are subject to a tensile loading. The mechanical properties ofthe anchor pins 28 allow them to support a substantial portion of theloading that would otherwise be taken by the solder balls 30. Note theregion 32 of the circuit board 22 that is held by the anchor pins 28 toconform to the shape of the package 20. As a result, none of the solderballs 30 experiences a stress large enough to cause immediate or fatiguefailure.

A second embodiment of the present invention method is illustrated inFIG. 8, which shows a side view of the IC package 20 and the circuitboard 22. The package 20 is anchored to the circuit board 22 by aplurality of metal straps 32. The metal straps 32 are disposed aroundthe perimeter, on all four edges, of the IC package 20. The metal straps32 are fused to four edges of the package 20 and the circuit board 22,and can be produced at the same time and by the same device as theelectrically connecting solder balls 30. It is important to note thatthe metal straps 32 do not provide any electrical connection between thepackage 20 and the circuit board 22. The exact quantity, dimensions,placement, and material of the metal straps 32 are determined based onrelevant design parameters, such as expected manufacturing or operatingtemperature range and external loading.

A third embodiment of the present invention method is illustrated inFIG. 9, which shows a side view of the IC package 20 and the circuitboard 22. The package 20 is anchored to the circuit board 22 by aplurality of redundant solder balls 34. The solder balls 34 are disposedat the four corners of the IC package 20. These mechanically connectingsolder balls 34 are fused to the bottom of the package 20 and thecircuit board 22, and can be produced at the same time and by the samedevice as the electrically connecting solder balls 30. As similar withthe previously described embodiments, the redundant solder balls 34 donot provide any electrical connection between the package 20 and thecircuit board 22. The exact quantity, diameter, placement, and materialof the redundant solder balls 34 are determined based on relevant designparameters, such as expected manufacturing or operating temperaturerange and external loading.

A fourth embodiment of the present invention method is illustrated inFIG. 10, which shows a bottom view of the IC package 20. For clarity,the circuit board 22 is not shown in FIG. 10. A continuous strip ofsolder 36 is disposed along the perimeter of IC package 20 tomechanically connect the package 20 to the circuit board 22. The solderstrip 36 can be produced at the same time and by the same device as theelectrically connecting solder balls 30. No electrical connectionbetween the package 20 and the circuit board 22 is provided by thesolder strip 36. The exact length, diameter, location, and material ofthe strip of solder 36 are determined based on relevant designparameters, such as expected manufacturing or operating temperaturerange and external loading. Alternatively, the solder strip 36 need notbe continuous and may have interruptions in its length.

In summary, the described method of attaching an IC package to a circuitboard offers a strengthened mechanical connection. In contrast to theprior art, the present invention anchors support a sufficient share of athermally induced or direct mechanical load, both of which may besimultaneous and cyclic, to prevent solder balls from becomingoverstressed and cracking. This enhanced stiffness package attachmentmethod will lead to less mechanical failures of solder balls.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for attaching an integrated circuit (IC) package to acircuit board, the IC package having a plurality of electrical contacts,the plurality of electrical contacts being disposed on a major surfaceof the IC package in an arrangement having a perimeter, the methodcomprising: positioning the IC package relative to the circuit board,such that the major surface of the IC package is adjacent to a majorsurface of the circuit board; electrically connecting the IC package tothe circuit board through the plurality of electrical contacts; andattaching at least one metal strap directly to the IC package tomechanically attach the IC package to the circuit board; wherein themetal strap does not provide an electrical contact between the ICpackage and the circuit board.
 2. The method of claim 1 wherein themetal strap is disposed at a location outside of the perimeter of theplurality of electrical contacts.
 3. The method of claim 2 wherein themajor surface of the IC package has a rectangular shape, and thearrangement of the plurality of electrical contacts is a grid-likearray.
 4. The method of claim 3 wherein metal straps are disposed atfour corners of the major surface of the IC package.
 5. The method ofclaim 3 wherein metal straps are disposed along four edges of the majorsurface of the IC package.
 6. The method of claim 3 wherein metal strapsare disposed on edges of the IC package.
 7. The method of claim 1wherein the electrical contacts are solder balls.
 8. The method of claim1 wherein the electrical contacts are pins.